In a digital logic circuit a desirable operating characteristic is a high switching speed from one logic state to another. However, other requirements, such as power dissipation, may constrain high speed designs. Digital signal transmission imposes such constraints; however, high transmission rate requires high switching speeds for the logic circuits transmitting and receiving the logic signals.
One type of data transmission is differential data transmission in which the difference in voltage levels between two signal lines form the transmitted signal. Differential data transmission is commonly used for data transmission rates greater than 100 kilobaud over long distances. Noise signals shift the ground level voltage and appear as common mode voltages. Thus, the deleterious effects of noise are substantially reduced.
To standardize such data transmission, various standards have been promulgated. These standards raise problems which must be overcome.
For example, one such standard is the recommended standard 422, RS422, which is defined by the Electronics Industry of America, EIA. This standard permits data rates up to 10 million baud over a twisted pair of signal lines. Driver circuits, i.e., circuits which place signals on the lines, must be capable of transmitting a minimum differential signal of two volts on the twisted pair line which terminates in 100 ohms resistance.
This requirement places a difficult constraint on the driver circuit where the circuit is powered by a voltage supply fairly close to ground, such as +5 volts, a commonly used voltage supply for integrated circuits. The driver circuit must switch in a 2-volt swing within the 5-volt differential between the voltage supply and ground. The switching speed of the driver circuit must remain high to permit a high rate of data transmission and the circuit must be able to handle the larger currents to drive the digital signals onto the signal lines.
Another problem with RS422 is that the twisted line pair is often used as a bus to which multiple drivers, sources of signal, are attached. When multiple drivers are connected to a common bus, only one driver may transmit data at a time. The remaining drivers should be in a high impedance state so as not to load the bus. Since large positive and negative common mode signals may appear at the driver output terminals connected to a bus system, the maintenance of a high impedance over a wide common mode voltage range and independent, whether the driver is powered or not, is desirable.
Still another problem is the possibility of excessive currents through the driver circuit. When multiple driver circuits are connected to a bus, the different driver circuits may be grounded at a different voltage levels. This produces a differential in the common mode voltage at the output terminals of the driver circuits if more than one of these circuits try to communicate over the bus at one time. The voltage differential produces a D.C. current through the driver circuit. Unless the current is limited in some way, the D.C. current may raise the temperature of the circuit so high that the driver circuit is damaged, a common danger for integrated circuits.
Furthermore, undesired heat may be produced by common mode voltage differentials, heat may also be produced in other ways. The driver circuit should be protected from excessive heat.